3 Global Winds and Local Winds

Transcription

1 CHAPTER 15 3 Global Winds and Local Winds SECTION The Atmosphere BEFORE YOU READ After you read this section, you should be able to answer these questions: What causes wind? What is the Coriolis effect? What are the major global wind systems on Earth? National Science Education Standards ES 1j What Causes Wind? Wind is moving air caused by differences in air pressure. Air moves from areas of high pressure to areas of low pressure. The greater the pressure difference, the faster the air moves, and the stronger the wind blows. You can see how air moves if you blow up a balloon and then let it go. The air inside the balloon is at a higher pressure than the air around the balloon. If you open the end of the balloon, air will rush out. STUDY TIP Underline Each heading in this section is a question. Underline the answer to each question when you find it in the text. 1. Define What is wind? What Causes Differences in Air Pressure? Most differences in air pressure are caused by differences in air temperature. Temperature differences happen because some parts of Earth get more energy from the sun than others. For example, the sun shines more directly on the equator than on the poles. As a result, the air is warmer near the equator. The warm air near the equator is not as dense as the cool air near the poles. Because it is less dense, the air at the equator rises, forming areas of low pressure. The cold air near the poles sinks, forming areas of high pressure. The air moves in large circular patterns called convection cells. The drawing on the next page shows these convection cells. 2. Identify On the drawing, label the high-pressure area with an H and the lowpressure area with an L. 3. Explain Why isn t all the air on Earth at the same temperature? Interactive Textbook 289 The Atmosphere

2 4. Describe Is air rising or sinking in areas of high pressure? Convection Cells Low pressure High pressure High pressure 90 S Low pressure 0 Equator High pressure 90 N Low pressure High pressure 5. Identify What are the three main global wind belts? What Are the Major Global Wind Systems? Global winds are large-scale wind systems. There are three pairs of major global wind systems, or wind belts: trade winds, westerlies, and polar easterlies. Trade winds are wind belts that blow from 30 latitude almost to the equator. They curve to the west as they blow toward the equator. Westerlies are wind belts that are found between 30 and 60 latitude. The westerlies blow toward the poles from west to east. Most of the United States is located in the belt of westerly winds. These winds can carry moist air over the United States, producing rain and snow. Polar easterlies are wind belts that extend from the poles to 60 latitude. They form as cold, sinking air moves away from the poles. In the Northern Hemisphere, polar easterlies can carry cold arctic air over the United States. This can produce snow and freezing weather. Wind belt Location (latitude) Toward the equator or toward the poles? Trade winds 0 to 30 toward the equator Westerlies 6. Describe Fill in the blanks in the table. 60 to 90 The figure on the next page shows the locations of these different wind belts. Notice that the winds do not move in straight lines. The paths of the wind belts are controlled by convection cells and by the Earth s rotation. Interactive Textbook 290 The Atmosphere

3 The trade winds meet and rise near the equator in a region known as the doldrums. The wind in the doldrums is very weak. The region between the trade winds and the westerlies is known as the horse latitudes. Here, cool air sinks, creating a region of high pressure. The winds here are very weak. 30 N 30 S 60 N Horse latitudes Doldrums Polar easterlies Horse latitudes 60 S Polar easterlies 90 N Trade winds Trade Trade winds winds Westerlies Westerlies 90 S There are three pairs of major global wind belts on Earth: the polar easterlies, the westerlies, and the trade winds. Cool air Warm air Wind direction STANDARDS CHECK ES 1j Global patterns of atmospheric movement influence local weather. Oceans have a major effect on climate, because water in the oceans holds a large amount of heat. Word Help: major of great importance or large scale 7. Explain Use the map to explain why surface winds are generally very weak near the equator. Why Do Global Winds Curve? Remember that pressure differences can cause air to move and form winds. If Earth did not rotate, these winds would blow in straight lines. However, because Earth does rotate, the winds follow curved paths. This deflection, or curving, of moving objects from a straight path because of Earth s rotation is called the Coriolis effect. As Earth rotates, places near the equator travel faster than places closer to the poles. This difference in speed causes the Coriolis effect. Wind moving from the poles to the equator is deflected to the west. Wind moving from the equator to the poles is deflected east. 8. Describe How does Earth s rotation affect the paths of global winds? Earth s rotation Path of wind without Coriolis effect Approximate path of wind The Coriolis effect causes wind and water to move along curved paths. 9. Apply Ideas If air is moving south from California, which way will it tend to curve? Interactive Textbook 291 The Atmosphere

4 10. Identify In what two layers of the atmosphere are the jet streams found? What Are Jet Streams? The polar easterlies, prevailing westerlies, and trade winds are all winds that we feel on the ground. However, wind systems can also form at high altitude. Jet streams are narrow belts of very high-speed winds in the upper troposphere and lower stratosphere. They blow from west to east all the way around the Earth. Jet streams can reach speeds of 400 km/h. Pilots flying east over the United States or the Atlantic Ocean try to catch a jet stream. This wind pushes airplanes along, helping them fly faster and use less fuel. Pilots flying west try to avoid the jet streams. The global wind systems are always found in about the same place every day. Unlike these global wind systems, jet streams can be in different places on different days. Because jet streams can affect the movements of storms, meteorologists try to track the jet streams. They can sometimes predict the path of a storm if they know where the jet streams are. 11. Infer Why would a pilot flying across North America take a different route on Tuesday than on Monday? Jet streams form between hot and cold air masses. Unlike the other wind systems, jet streams are found in slightly different places every day. Interactive Textbook 292 The Atmosphere

5 What Are Local Winds? Most of the United States is in the belt of prevailing westerly winds, which move from west to east. However, you ve probably noticed that the wind in your neighborhood does not always blow from the west to the east. This is because global winds are not the only winds that blow. Local winds are also important. Local winds are winds that generally move over short distances and can blow from any direction. Like the other wind systems, local winds are caused by differences in temperature. Many of these temperature differences are caused by geographic features, such as mountains and bodies of water. The figure below shows how water and mountains can affect local winds. Critical Thinking 12. Compare Describe one difference between global winds and local winds. Day During the day, the ocean heats up more slowly than the land. A cool sea breeze blows from the ocean to the land. The land heats up quickly. The air above it warms and rises. This produces an area of low pressure, so the sea breeze can blow over the land. Say It Share Experiences Have you ever been in a very strong wind? In groups of two or three, discuss the strongest or worst wind you ve ever been in. Night At night, the ocean cools off more slowly than the land. The warmer air over the ocean rises, producing an area of low pressure. The land breeze can blow over the ocean. The land cools off quickly. The air above it cools and sinks toward the ocean, producing a cool land breeze. 13. Identify In the figures, label the high-pressure areas with an H and the low-pressure areas with an L. MOUNTAIN BREEZES AND VALLEY BREEZES Mountain and valley breezes are other examples of local winds caused by geography. During the day, the sun warms the air on mountain slopes. The warm air rises up the mountain slopes, producing a warm valley breeze. At night, the air on the slopes cools. The cool air moves down the slopes, producing a cool mountain breeze. Interactive Textbook 293 The Atmosphere

6 Section 3 Review NSES ES 1j SECTION VOCABULARY Coriolis effect the curving of the path of a moving object from an otherwise straight path due to the Earth s rotation jet stream a narrow band of strong winds that blow in the upper troposphere polar easterlies prevailing winds that blow from east to west between 60 and 90 latitude in both hemispheres trade winds prevailing winds that blow from east to west from 30 latitude to the equator in both hemispheres westerlies prevailing winds that blow from west to east between 30 and 60 latitude in both hemispheres wind the movement of air caused by differences in air pressure 1. Identify The drawing below shows a convection cell. Put arrows on the cell to show which way the air is moving. Label high pressure areas with an H and low pressure areas with an L. Label cold air with a C and warm air with a W. 2. Identify Which global wind system blows toward the poles between 30 and 60 latitude? 3. Explain Why does wind tend to blow down from mountains at night? 4. Apply Concepts Would there be winds if Earth s surface were the same temperature everywhere? Explain your answer. Interactive Textbook 294 The Atmosphere

CHAPTER 6 3 Global Winds and Local Winds SECTION The Atmosphere BEFORE YOU READ After you read this section, you should be able to answer these questions: What causes wind? What is the Coriolis effect?

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